Were you holding your foot on the floor trying to get it to go faster? usually backing off a tad on the throttle works wonders, allows it to change up, it does take a bit of getting used to.
Have you checked gearbox oil level? engine warm & running, brakes on, range through the gears & into P (some say D, very little difference, can be easier to see the level in D, less turbulence), dipstick under the drivers seat angling to the rear.
Check the colour & condition, should beo red/orange (DIII ATF) if it smells like burnt toast or is going black, you may have issues. Could be worth changing the fluid & cleaning the filter anyway if you don't know when it was last done -
http://www.igmaynard.co.uk/bongo/forum/ ... =3&t=30096 or there's a factsheet in the members area.
You can't polish a turd - but you can roll it in glitter.